Render reason for invite rejection.

[Half-Shot]

Fixes #29256

This is a trivial change that will massively help out bot developers such as myself, so that users can see why the bot told them to go away.

Checklist

• Tests written for new code (and old code if feasible).
• [N/A] New or updated public/exported symbols have accurate TSDoc documentation.
• Linter and other CI checks pass.
• I have licensed the changes to Element by completing the Contributor License Agreement (CLA)

[dbkr]

Looks fine code-wise, I think we can probably skip product / design for this as it's fairly obvious. My one concern is T&S so I've asked them to check (probably fine but I figure they would at least want to know).

[dbkr]

T&S say yes